The simulation of vibrations experienced by patients during helicopter winching and retrieval

Grabau, Peter John (2016) The simulation of vibrations experienced by patients during helicopter winching and retrieval. PhD thesis, James Cook University.

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Due to the potential for Decompression Illness (DCI) to cause permanent or fatal injuries patients experiencing significant DCI symptoms while SCUBA diving on offshore reefs often require urgent helicopter aeromedical transfer to a specialist hyperbaric treatment facility. During these transfers patients are placed supine in a rescue litter (winch basket) for winching up to the helicopter and remain supine in the litter for the duration of the flight. The litter is secured directly to the cabin floor exposing patients to the high level of vibrations generated by the helicopter.

The risks associated with exposing patients to the high levels of vibration experienced during helicopter retrieval are not understood and consequently there is no existing policy or recommendation for the safe transport of patients with respect to the levels of vibration experienced during helicopter transport. This is despite research showing that the human body is vulnerable to the range of vibration frequencies commonly found in helicopters. A number of researchers have highlighted the potential risks and recommended studying the effects in clinical laboratory trials however this has not transpired due to the unavailability of a suitable vibration simulator capable of reproducing the vibrations experienced by a patient during actual retrieval.

Helicopter vibrations are characterised as three-dimensional, tonal, time-varying and broad spectrum with a high root-mean-squared (Armstrong) average level and a high crest factor making them difficult to generate artificially. Consequently recommended practice for vibration testing of helicopter instrumentation is to use real pre-recorded vibration data to drive the simulator however this data is not publicly available and will need to be recorded as part of the development of a simulator. The ideal simulator for use in clinical trials studying the effects of helicopter vibrations on retrieval patients would therefore accommodate a test subject supine in a rescue litter and reproduce pre-recorded real vibrations.

This thesis presents the results of research into the feasibility of creating a helicopter patient retrieval vibration simulation system which both records real patient vibration data during actual helicopter winching and retrieval and reproduces the vibration experience in a laboratory vibration simulator that can be located adjacent to a hyperbaric chamber. The research showed that the system was feasible and resulted in two operational systems. The first records tri-axial vibrations at the head, waist and feet of a patient supine in a rescue litter during winching and retrieval and the second is a vibration simulator which reproduces vertical axis vibrations for a patient supine in a rescue litter. The system is initially being applied to clinical trials studying the effects of transport induced vibrations on persons suffering from Decompression Illness (DCI). There are four main contributions from this thesis:

Firstly, the development of a vibration measurement system which attaches to the underside of a helicopter rescue litter and records real patient vibration data during all flight modes including winching. The system is non-invasive allowing the rescue litter to be safely moved into the cabin at the top of a winching operation. Vibrations are sensed using three tri-axial Micro Electro Mechanical Systems (MEMS) accelerometer sensors mounted in a protective void under the rescue litter. The sensors are aligned with the lateral, longitudinal and vertical axes and situated under the patients head, waist and feet. Simultaneous sampling of the nine vibration channels allows for comparison of the vibrations at the head, waist & feet and reconstruction of the vibrations in three dimensions. The system has been tested in a Bell 412 rescue helicopter and the recorded vibrations have been verified against manufacturer's data and vibration levels published in the literature.

Secondly, is the use of low-cost Micro Electro Mechanical Systems (MEMS) accelerometers as vibration sensors. MEMS accelerometers offer the advantages of being low cost, low power and low profile however their use as vibration sensors is not established as they require individual calibration by the user and some researchers have reported non-ideal frequency response characteristics. Their low profile makes them ideal for this application as it allows the sensors to be safely mounted in the protective void under the rescue litter making the system less vulnerable to damage during movement of the litter. The thesis presents the detailed development of the MEMS based sensors including the procedure that was used for their calibration.

Thirdly, is an algorithm which allows high speed continuous streaming of data to an SD card from a low-power 8-bit microcontroller by means of a serial interface. The algorithm uses three techniques to improve the data streaming rate. It begins at initialisation by erasing sufficient space on the SD card to store all the data for the test and then creating a single contiguous test file in the file allocation table linked list. During recording data is then written in whole segments to the next physical SD card memory segment without reference to the file allocation table. The algorithm is a critical part of the vibration measurement system enabling non-volatile storage of the measured data.

Finally, the design and development of a vibration simulator capable of supporting a subject lying supine in a rescue litter and reproducing the real vibrations recorded by the vibration measurement system. Powerful electrodynamic shakers provide the vibratory force ensuring high fidelity of reproduction. The simulator will allow clinical trials into the effects of vibrations on patients during helicopter winching and retrieval.

Item ID: 44642
Item Type: Thesis (PhD)
Additional Information:

air ambulances; helicopter rescues; helicopter winching; medevac; medivac; patient retrieval; patient transport; vibration analysis; vibration simulation; vibrations

Date Deposited: 11 Aug 2016 04:28
FoR Codes: 09 ENGINEERING > 0906 Electrical and Electronic Engineering > 090604 Microelectronics and Integrated Circuits @ 40%
09 ENGINEERING > 0906 Electrical and Electronic Engineering > 090602 Control Systems, Robotics and Automation @ 40%
09 ENGINEERING > 0906 Electrical and Electronic Engineering > 090609 Signal Processing @ 20%
SEO Codes: 88 TRANSPORT > 8803 Aerospace Transport > 880302 Air Passenger Transport @ 50%
92 HEALTH > 9202 Health and Support Services > 920299 Health and Support Services not elsewhere classified @ 50%
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